1. Field of the Invention
The present invention relates to an apparatus and method for detecting a correlation which outputs a correlation signal indicative of a correlation between a transmitted signal which has been spread in spectrum and a spread code signal, and a spectrum despread apparatus and receiver having the apparatus for detecting a correlation.
2. Description of the Prior Art
CDMA (Code Division Multiple Access) system attracts attention as a multiple access system in a mobile communication system including base stations and transmission/reception terminals as portable mobile stations, because the CDMA system has a possibility to drastically increase subscriber capacity. In the CDMA system, a signal to be transmitted is spread in spectrum with a spread code signal such as a M-sequence code signal and a Gold sequence code signal before transmission from a transmitting apparatus, which is a base station or a transmission/reception terminal, and a transmission signal received by a receiving apparatus, which is the transmission/reception terminal or the base station, is despread by the same spread code signal as the transmitting apparatus to produce a decoded signal.
In order to despread a transmission signal with a spread code signal in a spectrum despread apparatus of a receiving apparatus, it is necessary to generate a spread code signal having a sequence and phase which are the same as the transmission signal. The phase of the spread code signal which has spread the transmission signal is detected by detecting a peak timing of an output of a correlation detecting apparatus.
According to a signal format of a W-CDMA (Wideband Code Division Multiple Access) system proposed by ARIB (Association of Radio Industries and Businesses) as shown in FIG. 6, Perch Channel's one frame having a period of 10 msec is divided into 16 slots and each slot is divided into 10 symbols. A Search Code is assigned to the first symbol of each slot. The Search Code is a code common among all the transmission/reception terminals and composed of 256 chips. A correlation detecting apparatus of each transmission/reception terminal outputs a correlation signal in one slot time at a minimum by using the Search Code. The correlation detecting apparatus outputs a correlation signal as shown in FIG. 7 as a phase detection signal. In addition, the correlation detecting apparatus oversamples each chip. An oversampling frequency thereof is, for example, a double or quadruple of a chip rate.
Formerly, the Search Code consisting of 256 chips was of 256 period. ARIB, however, has proposed the Search Code of L×M period, where L×M=256. The Search Code of L×M period is a Search Code which repeats a spread signal of a period of L by M times. The values of L and M are integers larger than one. The values of L and M are, for example, 16 and 16, respectively. The Search code of L×M is inverted or not inverted in a unit of the value of M in accordance with a prescribed rule. There may be an extreme rule which does not invert the Search Code at all.
The correlation detection apparatus proposed by ARIB, AIF/SWG2-28-18, Cell Search Scheme for 1st and 2nd stage, ST8 as shown in FIG. 8 comprises L-chip accumulator 901, shift register 902 consisting of D-type flip-flops as many as L×(M−1)×N, adder 903 having inputs as many as M, and multiplier 904 as many as M.
L-chip accumulator 901 may be, for example, a matching filter or a correlator bank.
As shown in FIG. 5, a matching filter as an example of L-chip accumulator 901 comprises shift register 201 consisting of D-type flip-flops as many as (L−1)×N, multipliers 203 as many as L which multiply signals derived from every N-th taps of shift register 201 with coefficients γi (i=1, 2, . . . , L), and adder 202 which sums up the outputs of multipliers 203. The matching filter takes a form of a transversal filter.
A bit width of an input of L-chip accumulator 901 is, for example, 8. A bit width of an output of L-chip accumulator 901 is 12 if the bit width of the input of L-chip accumulator 901 is 8 and the number L of inputs of adder 202 is 16.
Next, the operation of the correlation detecting apparatus will be explained with reference to FIGS. 5 and 8.
A transmission signal which has been oversampled into N samples per chip is inputted to L-chip accumulator 901. L-chip accumulator 901 adds/subtracts samples as many as L and outputs an intermediate correlation signal at each clock tick of the oversampling frequency.
The intermediate correlation signal and delayed intermediate correlation signals which are derived from every L×N-th tap of shift register 902 are inputted to multipliers 904. The coefficients βm(m=1, 2, . . . , M) of multipliers 904 are determined in accordance with the Search Code with L×M period. Adder 903 sums up outputs of multipliers 904 to output the sum thereof as a final correlation signal.
However, the correlation detecting apparatus as shown in FIG. 8 has disadvantages as follows:
A first disadvantage is that shift register 902 is composed of a large number of D-type flip-flops as many as L×(M−1)×N. This causes an increase in circuit scale.
A second disadvantage is that input the data and output data of the D-type flip-flops, as many as L×(M−1)×N, constituting shift register 902 change at each clock tick of the oversampling frequency. This causes an increase in necessitative power consumption.
The above disadvantages are serious for a portable type of a transmission/reception terminal which operates with a battery if the correlation detecting apparatus of FIG. 8 is incorporated therein.